1. Technical Field
The present invention relates to a liquid-crystal display device comprising a polarizing plate and a retardation film, and in particular to a VA (vertical alignment)-mode liquid-crystal display device.
2. Background Art
It is known that a VA-mode liquid-crystal display device can realize a wide viewing angle, or that is, can have improved display characteristics, as comprising polarizing plates each disposed on and below a liquid-crystal cell with their absorption axes crossing vertically to each other, and having an optically biaxial retardation film disposed between each polarizing plate and the liquid-crystal cell (for example, Japanese Patent 3330574).
FIG. 7B shows a cross section of a liquid-crystal display device in which the above-mentioned, optically-biaxial retardation films having the same optical anisotropy are used on and below a VA-cell; and FIG. 7A shows the polarization state of the light that passes through the layers of the device, as arrows on a Poincare sphere. In this system, a film of the same type may be disposed on and below the VA cell to enable optical compensation, therefore having the advantage in that the mass-scale production cost is reduced. However, in this system, in case where Δnd of the liquid-crystal cell changes, it is necessary to re-plan the optimum retardation in plane Re and retardation along thickness direction Rth of the biaxial retardation films every time with the change (for example, as shown in FIGS. 8A and 8B, it is necessary to replace Retardation Film X with Retardation Film X′ having different optical characteristics). Variously changing Δnd of the liquid-crystal cell is under investigation for the purpose of power saving and rapid response; and trying to compensate the system employing such a different liquid crystal cell by two retardation films having the same optical anisotropy, it is necessary to re-plan the optimum optical characteristics of the retardation films for each of liquid-crystal cells having a different value of Δnd, and it is also necessary to re-plan the production line.
On the other hand, a system has been proposed for reducing the light leakage depending on the variation of wavelength of light, or that is, for reducing the color shift in undesirable coloration in blue or red, which is observed in the oblique directions in the black state of liquid-crystal display devices. In the proposed system, used are two retardation films, concretely, an optically-positive monoaxial film (generally A plate) and an optically-negative monoaxial film (generally C plate), having a specific wavelength dispersion characteristics of retardation (for example, Japanese Patent 3648240). FIG. 9B shows a cross section of a liquid-crystal display device of a combination of A plate and C plate; and FIG. 9A shows the polarization state of the light that passes through the layers, as arrows on a Poincare sphere. In this system, it is unnecessary to change the optical characteristics of the A plate for optical compensation of the liquid-crystal cell of which the Δnd may change variously; and in this, only changing the optical characteristics of the C plate may be enough to satisfy the condition of the changing Δnd of the liquid-crystal cell (for example, as shown in FIGS. 10A and 10B, it is necessary to replace C plate with other C plate, C plate′, having different optical characteristics, but it is not necessary to replace A plate with other A plate). However, this system requires polarization change with the A plate to the vertical line that passes through the extinction point P, and for achieving such a polarization change, it is necessary to use the A plate having large Re and Rth. However, it is not easy to produce A plates having such optical characteristics. It is not also easy to produce on an industrial scale a film satisfying the optical characteristics required for C plate (in a precise sense thereof, its retardation in plane (Re) is zero and its Rth is large). This is because, in industrial-scale continuous production of films, in general, the produced films may have some Re in the machine (or transversal) direction. Such films having some Re are, in a precise sense thereof, optically biaxial films.